I also had this problem in the past and recoverd by creating a completely new catalogue which took quite a while (Bibble XMPs should exist).
Maybe a little bit stupid but:"how can one remove a single image from a catalogue?" (never realised that this is possible)
Take note of the folder where the picture lives: Edit|Version|Show master file location
Most often, you'll have many pictures in the same folder showing the same problem; it could be useful to select 'Link to Catalogs' in the Metadata Browser.
If you did not allow ASP to create .xmp files (which you should), write them for your files before
Remove the picture: Edit|Version|Remove VersionDon't select
'Move master file to trash', of course
Reimport the folder, or just those pictures using drag'n'drop (this would open a queue). Even if the rogue metadata are still there, you usually
can remove them at this point. If that doesn't work, you will have to edit by hand the xmp file.
You will lose history on those pictures, of course; but it's still much
faster than reimporting the whole catalog.